Transcription

what is it?

The mechanism by which one strand of the DNA molecule is used as a template to yield an RNA molecule

The gene is the region of the DNA that is transcribed

The RNA product may be mRNA, tRNA, snRNA, or another type of RNA

Gene: A hereditary unit occupying a specific position within the genome

The information content of genes is in the form of specific sequences of nucleotides along strands of DNA.

Similarities between DNA replication

Fundamentally similar chemical mechanism

Its polarity ( Direction of synthesis) addition of nucleotides to the -3 OH

Use of a template

Stages of initiation, elongation and termination

RNA

Generally one stranded

Ribose as the sugar

Bases used = U,C,A,G

DNA

Deoxyribose as the sugar

Double stranded

Bases used: T,C,A,G

FORMATION OF A PHOSPHODIESTER BOND = CONDENSATION REACTION

Difference between DNA replication

Transcription only use RNA polymerase, so does not require a primer

Generally involves a limited section of the DNA

Within a transcribed segment only one DNA strand serves as a template

Basic structure of a gene

The promoter = bases pair sequence that specifies where transcription begins

The coding sequence is a base - pair sequence. It is where the coding information a polypeptide chain is specified, if this is a gene for mRNA

The terminator is a sequence that specifies the end of RNA transcript

Flanking regions

starts at 5' ends at 3'

Leader length is variable

3' trailer aids stability, long sequences of Us cause instability

Template strand: Is what is used as a template strand in the synthesis of mRNA

Coding strand: is not used as a template, but is identical in sequence to mRNA / DNA U's are T's

Promoter sequences: A stretch of DNA sequence that lies upstream that is required to direct the inititation of transcription by an RNA Polymerase

RNA polymerase

bacteria have a single type of RNA polymerase that catalyses synthesis of mRNA, rRNAs and tRNAs encoded in the bacterial chromosome

Synthesis of RNA Transcript

Initiation

Elongation

Termination

RNA polymerase 2 Binding and initiation of transcription

Transcription factors mediate the binding of RNA polymerase and the initiation of transcription

The completed assembly of transcription factors and RNA polymerase 2 bound to a promoter is called a transcription initiation comple

Promoter called a TATA box is crucial in forming the initiation complex

RNA polyermase moves along the DNA, it untwists the double helix 10-20 bases at a time

Requires for the RNA polymerase enzyme to be released and also the RNA product to be released

Prokaryote termination

Rho dependent: Hexameric protein

interaction activates an ATPase of Rho that is associated with its movement along the RNA in the 3' direction until it unwinds RNA-DNA hybrid at the active site of RNA polymerase

Termination in Eukaryotes

RNA pol 1 genes trancription is stoppped using a termination factor

RNA pol 111 genes ends after transcribing a termination sequence

RNA pol 11: Connected mechanism of transcription termination and RNA 3' end processing,

Cells modify RNA after transcription

Enzymes in nucleus modify pre-mRNA before the genetic messages are dispatched to the cytoplasm

during processing both ends of the primary transcript are usually altered

Alteration of mRNA ends

3' end get a poly A tail

5' end gets a nucleotide 5'cap

Functions: Protect mRNA from hydrolytic enzymes - They help ribosomes attach to the 5'end - They seem to facilitate the export of mRNA

mRNA processing the 5'

7- methyl guanosine is added by a triphosphodiester bond - It provides binding for ribosome and also protects the 5' end

Introns: Long noncoding regions of nucleotides

Exons: are eventually expressed and translated into amino acid sequences

RNA Splicing removes introns and joins exons, creating a continuous coding sequence

Spliceosomes: Consist of a variety of proteins and several small nuclear ribonucleoproteins that recognise and splice sites